Surface mount solder assembly of leadless integrated circuit packages to substrates

Abstract

Described are a process for soldering at least one component having solder bumps to a substrate and a process for forming solder bumps on metal pads of an element, such as an IC package or substrate or both. The bumps are formed by stencil printing solder paste deposits on the metal pads, heating the solder paste deposits to reflow temperature of the solder in the solder paste deposits, and allowing the molten solder in each deposit to coalesce and during subsequent cooling solidify forming the bumps on the metal pads. The bumps are formed by conducting the stencil printing through apertures in an ultra-thick stencil, the apertures having trapezoidal crossection in the plane normal to the broad surfaces of the stencil with the top opening being smaller than the bottom opening and with the walls of the aperture sloping at an angle within a range of from 1 to 45 degrees from the vertical, the solder paste having a low tackiness and high metal loading, and the solder paste deposits covering an area which is equal to or exceeds an area of the metal pad in any ratio between 1.5:1 and 5:1. Bumps formed in this manner lead to the formation of reliable solder joints.

We claim: 
     
       1. A process of soldering at least one component to a substrate, said process comprising forming a plurality of solder bumps on pads of said at least one component, placing the component with solder bumps adjacent pads on the substrate, heating the bumps to a temperature sufficient to reflow said bumps, and allowing the bumps to resolidify joining the component to the substrate, said bumps are formed by stencil printing solder paste deposits on said pads of the component, heating the solder paste deposits to the reflow temperature of the solder in the solder paste, and allowing the solder to solidify forming the bumps on the pads of the component, wherein   said stencil printing is conducted through apertures in an ultra-thick stencil,   the apertures have trapezoidal crossection in the plane normal to the broad surfaces of the stencil, with the top opening of the aperture being smaller than the bottom opening and with the walls of the aperture sloping at an angle within a range of from 1 to 45 degrees from the vertical,   said solder paste has a low tackiness and high metal loading characteristics, and   said solder paste deposit covers an area which is equal to or exceeds an area of the metal pad in a ratio of from 1.5:1 to 5:1.   
     
     
       2. The process of claim 1, in which the stencil is 12-30 mils thick. 
     
     
       3. The process of claim 1, in which the area covered by the solder paste deposit is at least 3.5 times as large as the area of the pad. 
     
     
       4. The process of claim 1, in which the walls of the apertures slope 5-10 degrees from the broad surface of the stencil. 
     
     
       5. The process of claim 1, in which an effective diameter of the base of the trapezoid is greater than the height of the trapezoid in a ratio ranging from 1.5 to 5. 
     
     
       6. The process of claim 1, in which the solder powder is selected from the group consisting of Sn/Pb, Sn/Ag, Sn/Sb, Sn/Zn, Sn/Bi and Sn/Pb/Bi solder alloys and their combinations. 
     
     
       7. The process of claim 1, in which said solder powder is a Sn/Pb/Bi (43/43/14) solder powder. 
     
     
       8. The process of claim 1, in which said solder paste comprises solder particles and a flux vehicle, said flux vehicle comprises 1 to 10 wt. % of an organic acid, 1 to 5 wt. % of theological additive comprising a polymer having molecular weight greater than 50,000, 60 to 98 wt. % of solvent system, and 0 to 25 wt. % of further additives whose residue maintains high surface insulation. 
     
     
       9. The process of claim 8, in which said flux vehicle comprises 1-8 weight percent 4-hydroxybenzoic acid, 1-3 wt. % ethyl cellulose, 15-30 wt. % propyl-4-hydroxybenzoate, 0-25 wt. % of hydrogenized rosin gum, and the remainder being tripropylene glycol. 
     
     
       10. The process of claim 9, in which said flux vehicle comprises 6.0 wt. % 4-hydroxybenzoic acid, 1.5 wt. % ethyl cellulose, 25 wt. % propyl-4-hydroxybenzoate, 14 wt. % of hydrogenized rosin gum, and 53.5 tripropylene glycol. 
     
     
       11. The process of forming solder bumps on metal pads of an element, which comprises stencil printing solder paste deposits on the pads of the element,   applying heat to melt the solder, and   allowing the molten solder to solidify forming truncated spherical solder bumps of said pads, wherein   said stencil printing is conducted through apertures in an ultra thick stencil,   said apertures have trapezoidal crossection in the plane normal to the broad surfaces of the stencil, with the top opening of the aperture being smaller than the bottom opening and with the walls of the aperture sloping at an angle within a range of from 1 to 45 degrees from the vertical,   said solder paste has a low tackiness and high metal loading characteristics, and   said solder paste deposit covers an area which is equal to or exceeds an area of the metal pad in a ratio of from 1.5:1 to 5:1.   
     
     
       12. The process of claim 11, in which the stencil is 12-30 mils thick. 
     
     
       13. The process of claim 11, in which the area covered by the solder paste deposit is at least 3.5 times as large as the area of the pad. 
     
     
       14. The process of claim 11, in which the walls of the apertures slope 5 to 10 degrees from the broad surface of the stencil. 
     
     
       15. The process of claim 11, in which an effective diameter of the base of the trapezoid is greater than the height of the trapezoid in a ratio ranging from 1.5 to 5. 
     
     
       16. The process of claim 11, in which the solder powder is selected from the group consisting of Sn/Pb, Sn/Ag, Sn/Sb, Sn/Zn, Sn/Bi and Sn/Pb/Bi solder alloys and their combinations. 
     
     
       17. The process of claim 11, in which said solder powder is a Sn/Pb/Bi (43/43/14) solder powder. 
     
     
       18. The process of claim 11, in which said solder paste comprises solder particles and a flux vehicle, said flux vehicle comprises 1 to 10 wt. % of an organic acid, 1 to 5 wt. % of rheological additive comprising a polymer having molecular weight greater than 50,000, 60 to 98 wt. % of solvent system, and 0 to 25 wt. % of further additives whose residue maintains high surface insulation. 
     
     
       19. The process of claim 18, in which said vehicle comprises 1-8 wt. % 4-hydroxybenzoic acid, 1-3 wt. % ethyl cellulose, 15-30 wt. % propyl-4-hydroxybenzoate, 0-25 wt. % of hydrogenized rosin gum, and the remainder being tripropylene glycol. 
     
     
       20. The process of claim 19, in which said vehicle comprises 6 weight percent 4hydroxybenzoic acid, 1.5 wt. % ethyl cellulose, 25 wt. % propyl-4-hydroxybenzoate, 14 wt. % of hydrogenized rosin gum, and 53.5 wt. % tripropylene glycol.